1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In a conventional image forming apparatus such as a copying machine or a printer, a sheet whose one side has undergone image formation in an image forming unit is reversed, and conveyed again to the image forming unit via a reverse conveyance unit to perform image formation on the reverse side (two-sided printing). In such an image forming apparatus, there is a fear, at the time of two-sided printing, of the image formed on the sheet being deviated in the width direction, which is a direction orthogonal to the sheet conveyance direction.
This is due to the fact that, in the case of two-sided printing, the sheet conveyance path to be taken from the sheet feeding before the image formation on the second surface of the sheet is longer as compared with the case of one-sided printing. When the sheet conveyance path is thus long, the influence of minute misalignment of the conveyance rollers, distortion of the guide plate, etc. is added, sometimes resulting in gradual deviation of the sheet in the conveyance width direction.
In view of this, in the prior art, there is provided in the sheet conveyance path a detection sensor configured to detect a side edge position in the width direction of the sheet to detect the deviation amount in the width direction of the sheet. And, according to the deviation amount of the side edge position of the sheet thus detected by the detection sensor, the position of the latent image to be written to the photosensitive drum in the image forming unit is shifted, whereby the sheet is matched with the image forming position.
On the other hand, in recent image forming apparatuses, there is a demand for a further improvement in terms of user operability, in particular, of jamming handling property. In view of this, in more and more image forming apparatuses, the sheet conveyance apparatus is made detachable with respect to the image forming apparatus main body, and, when jamming has occurred, the sheet conveyance apparatus is drawn out to the front face, the rear face, or a side face of the apparatus main body, whereby the conveyance path is exposed to the exterior, thereby facilitating the jamming handling processing.
In such a detachable sheet conveyance apparatus, a detection sensor is arranged in the sheet conveyance path in the apparatus, and the position of a side edge position of the sheet is detected by this detection sensor. However, when the sheet conveyance apparatus is attached to the image forming apparatus main body, due to play between itself and the image forming apparatus main body and variation in component dimension, the sheet conveyance apparatus may be deviated in the sheet conveyance direction. In such cases, the detection sensor is also deviated with respect to the image forming apparatus main body, resulting in positional deviation between the detection sensor and the image forming unit.
In view of this, in a conventional image forming apparatus, the detection sensor is provided to be movable in the width direction; when detecting a sheet side edge position after the attachment of the sheet conveyance apparatus, the detection sensor is first moved toward the image forming apparatus main body to detect the position of a reference member provided in the image forming apparatus main body. And, after the position of the reference member is thus detected, the detection sensor is moved toward the sheet, whereby the sheet side edge position is detected (See Japanese Patent Application Laid-Open No. 2002-53246). Based on the movement amount of the detection sensor since the detection of the position of the reference member on the image forming apparatus main body side till the detection of the sheet side edge position, it is possible to obtain the actual sheet side edge position, in another words, the deviation amount of the sheet side edge position from the reference position. And, by adjusting the image writing position according to the deviation amount thus obtained, it is possible to correct the image forming position on the sheet.
As described above, in the conventional sheet conveyance apparatus and image forming apparatus as discussed in Japanese Patent Application Laid-Open No. 2002-53246, when detecting a sheet side edge position, the detection unit is first moved toward the reference member to detect the reference member. After this, to detect the side edge of the sheet, the detection unit is moved in the width direction of the sheet. However, when moving the sheet side end for detection after the movement of the detection unit toward the reference member to detect the reference member, it is necessary to secure a long movement distance for the detection unit for a case in which the sheet whose side edge position is to be detected is a sheet of a small width size. This involves a large-sized movement mechanism for moving the detection unit, resulting in an increase in the size of the apparatus.